text:
Is it normal to be scared of flying before becoming a pilot?
Are there any studies that have researched into fear of flying when not in the cockpit? The question arises from the fact that for some reason, I have found myself to be scared when on a plane as a passenger, but when at the commands the situation is totally different. This applies also to a lot of people when driving cars for example. Is this a normal human behaviour?

The aspect of fear when someone else is doing the flying/driving is completely normal, probably due to the stress caused by not being in control.  Loosely connected reading: https://www.psychologytoday.com/blog/conquer-fear-flying/201509/pilot-fearful-during-flight-passenger  This link is a bit "Pop" psychology but I'm very sure there will be more serious work somewhere.

Apparently, you're not the only one: https://youtu.be/IgbgEPqndVg?t=7m1s

I voted to leave open because human factors is part of aviation in my book.

It's interesting, I can relate this to cars. I am a driver with years of experience and hundreds of thousands of kilometers behind the wheel. I take my car to the track and drive quite aggressively and quick most of the time. However, I can't be a passenger on the track and even sometimes on normal roads with most of the people. I have a few that I trust though.

Quite handy for googling, it's named amaxophobia. Every possible phobia has a name, e.g. fear of figure 4 is tetraphobia. Should you be relieved of having no phobia at all, and logically fear to get one, then your already got a sneaky one named phobophobia. This may scare more than one, however I didn't check if the fear of phobophobia has a particular name though. Bottom line: phobias are very common reminiscences of (forgotten) past events, particularly from our childhood.

You should see my face when my sister is driving. It's hard to blink, you know.

Just because you are at the controls doesn't always mean you are in control of every situation. In a car, you have to share the road with a whole lot of other drivers; you can't count on every one of them to do the expected thing. Even in an aircraft, there may be situations over which you have limited control. The point is that it may be the illusion of being in control which calms your fears.rather than actually being in control.

I think its extremely normal. The fear of the unknown is very powerful. When you are in control, you generally know what is coming and aren't surprised.
Most pilots I know have no desire to sky dive. They love the freedom of flying, but falling uncontrolled is not enjoyable to them.
When I'm not PIC, sudden drops give me that put in the stomach feeling. When I'm in control, that NEVER happens.

it's not so much the fear of the unknown as it is about not being in control of things, or even having the illusion of being in control of things. Add a touch of claustrophobia when sitting in a metal tube with only a tiny window to your side rather than sitting in the front with a widescreen view of the world around you and the picture is complete.

I used to be terrified of flying. When I went on holiday I would book the day before so I didn't have to worry for weeks or months and then I would spend my two weeks worrying about having to get back on the flight. Then I went to the states and a year later decided to go back to the uk. I was so amazed how this B747 got off the ground with max all up weight on a very hot day that the next day I woke up and said "today I will learn to fly" and that I did. My instructor said you will love it or hate it, he was right I loved it. Now although I am a pilot, I am still a very very nervous passenger and I have come to realise the reason is as a passenger I have no control and I'm not alone. Many other pilots I know have said the same thing, it's just like Dr's don't make good patients. No I am not a control freak lol


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Why do DVORs and CVORs rotate in different directions?
I am currently studying both systems and I have the following doubt: why does DVOR rotate counterclockwise as opossed to CVOR? Could it be because the reference and variable signal are "swapped" between both of them?

Basically yes. What matters is how the two signals are rotated relative to each other.

So in this case, I can not understand how the signals can rotate relative to each other only variating the rotating direction

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text:
Is this exam question on traffic patterns wrong?
I just don't know the difference between Left-hand traffic and Right-hand traffic.
I thought Left-hand traffic uses when land to the East and right-hand traffic for when land to the west. Is anyone help me out?
Also, I got a question from the book about traffic patterns and I think the question is wrong... If above all things are true.

The trick to decoding the segmented circle is to understand it shows only base-to-final turns.

For left-hand traffic you make left hand turns no matter how the runway is oriented.

The north/south (360/180) rectangular patterns are to the west of the field, the east/west (90/270) patterns are to the north of the field.  Left pattern for 360 landing.  Right pattern for 270 landing.

Left and right refer to the direction of turns in the traffic patterns. In a Left hand traffic pattern, there are only left hand turns, and from the pilots perspective the runway is always on his/her/their left hand side. In a right hand traffic pattern, it's all turns are to the right, and the runways is to the pilot's on right.

Using left or right hand pattern has nothing to do with the direction of the runway.
Left hand pattern (all turns to the left) is the standard. It is the normal way to fly a traffic pattern if there is no specific reason to fly a right hand pattern (all turns to the right).
Most airfields allow both patterns. Reasons to prohibit either of patterns might be such as noise abatement, high terrain or other obstacles on one side of the runway.

I'm not so sure about the claim that "most airfields allow both patterns." At nontowered fields there is always a "correct" pattern - left by default, right traffic if so specified in the A/FD or by the traffic pattern indicator. At towered fields the local controller will assign the pattern based on local procedures. So you're never really free to choose. (That said, I've never heard of enforcement action for flying the wrong pattern unless you cause an accident... usually the worst that happens is a snarky base operator snips at you over the UNICOM.)

@TypeIA oh, this is interesting, I thought any pattern is allowed unless specifically prohibited, and right pattern always requires clearance, or announcement if no ATC is provided.

AIM 4-3-4(b) 5. c. "When approaching for landing, all turns must be made to the left unless a traffic pattern indicator indicates that turns should be made to the right."

Thanks @TypeIA, my memory seems to have failed me

@Jpe61 Note that left turns are by regulation, it’s not just in the AIM. See 91.126(b) for class G, which is referenced in the following sections for other airspace classes too.

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What is the lift coefficient of the B767-400ER?
Can anyone tell me what is the lift coefficient of B767-400ER please? I have searched it in Boeing website and Delta Airlines website but still couldn't find the data of the aircraft performances. Where could I find the detailed specifications of the aircraft?

Which lift coefficient are you looking for? Maximum? In cruise? At take-off rotation? Since the 767 can have very different weights and fly at very different speeds, the lift coefficient will also vary.

Similar question: How to calculate the Lift Coefficient for the A320? Going into more detail about what coefficient of lift means How do we derive Lift Coefficient of an airplane's wing?

There's an article on drag polar estimation for high subsonic aircraft on this link


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Does the Enroute Phase begin at a 30NM distance from the departure airport?
ICAO's Doc 8168 Volume II ("Aircraft Operations") states:

Part III "Performance-Based Navigation Procedures"
Section 1 "Underlying Principles"
Chapter 1 "RNAV Concepts"
1.2 "Flight Technical Error"
Table III-1-1-2. Phase of flight [...] En route (greater than or equal to 56 km (30 NM) from departure
or destination ARP. [...] Terminal (SIDs, STARs, initial and
intermediate approaches less than 56 km (30 NM) of the ARP).

A similar categorization can also be found in FAA's Order 8260.58A ("PBN Instrument Procedure Design"), which states:

1-2-5 "Obstacle Evaluation Area (OEA) and Flight Path construction"
b (2). Width. [...] For the en route transition, legs that cross a point 30 NM from ARP, the OEA primary area width expands or tapers at a rate of 30 degrees relative to course to the appropriate width [...] A departure or missed approach segment designed to cross a point 30 NM from the ARP more than once changes to RNAV 2/RNP 2/A-RNP (en route mode) and remains expanded when it crosses the boundary the first time.

They way I interpret the above instructions is that a departure procedure should end (either explicitly or via vectoring) at a 30 NM distance from the departure airport, where optionally the enroute transitions of the SID will guide the aircraft and prepare it for the cruise flight phase.
On the other hand, take FINZZ TWO where the distance between LGA airport and FINZZ is less than 15 NM, something that contradicts the aforementioned convention of 30 NM between the airport reference point and the start of the enroute mode.
Could somebody help me realize what I'm getting wrong?

You are not getting anything wrong, I think you are simply overthinking something that isn’t actually important.  The 30nm distinction is somewhat arbitrary, and presumably captures most arrival and departure procedures.  There is nothing magical or binding in this number, and there is nothing pilots will do differently when flying a published route or procedure based on distance or phase of flight.
For example, let’s say the last point on a published departure procedure is less than 30nm from your departure airfield, and the next point on your filed route of flight is outside of 30nm.  Your question implies that there would be confusion as to whether you would be in the terminal phase or the enroute phase of flight as you flew between these two waypoints, is that basically correct?  My point is that this distinction does not matter.  You will not fly the aircraft any differently if it is one or the other, you will simply navigate to the next point on your route while climbing or maintain assigned altitude.
Now there are some things that are accomplished during arrival and departure phases that may be worth discussing to clarify things and satisfy your curiosity on the subject.  However, most of them are done as a result of passing a certain altitude.  For example, at a certain altitude during climb out power may be adjusted for noise abatement and efficiency.  Passing through 18,000 feet altimeters will be adjusted to 29.92 and perhaps switched into a different mode.  This will be captured as part of a climb checklist.  Reaching cruise altitude another checklist will be accomplish that will include things like adjusting cruise power, checking cabin pressurization, etc.
Preparing for arrival similar descent and approach checklists are executed that will include obtaining weather and local altimeter setting, approach briefing, etc.  Passing 18,000 on the descent altimeters will be set to local setting, passing 10,000 speed will need to be slowed to 250KIAS.  Checklists can be triggered by either altitude or distance from the destination, but again the distinction between exiting the enroute phase and entering the terminal phase is not really that important.  The thing that matters is understanding when you are established on the published procedure you have been cleared for.  The actual distance from the airport where this actually occurs is of no real concern as long as you fly what you are cleared for.
If IFR there will also be a hand off at some point between the Air Route Traffic Control Center and the local approach controller, but this will likely be tied to altitude as well.  Also, you may be handed from one approach controller to another, but this will occur as you cross sector boundaries and is based more in geographic radar coverage and not a concentric range ring from the airport.
Finally, most modern GPS navigation systems will automatically change the sensitivity of the course deviation indicator in different phases of flight with the least sensitivity in enroute phase, more in terminal, and greater yet in approach phase.  (This is basically what the section you quoted above is referring to)  However, they will cycle as the aircraft passes the first or last waypoint of a procedure that is loaded into the system and not at some distance in between.  In other words, if you are navigating to the first point of a STAR or instrument approach the system ought to retain enroute CDI sensitivity until this point is reached.  The upon passing the waypoint it will cycle to the greater sensitivity of the terminal or approach phase.  For this reason is it important to understand what phase of flight your GPS considers you to be in, but know that it will not cycle at 30nm or any other arbitrary distance in between, but at the waypoint where the transition occurs.
To reiterate, there isn’t a specific checklist or different way of flying related to the transition between terminal and enroute phase of flight, nor is there any such thing as a “no man’s land” in between phases if a point on a SID or STAR is within or outside of the 30nm distance referenced in your question.

Thank you, your answer was very helpful. Sorry for the late response.

You are welcome.  FYI, I added a brief paragraph on ATC after a re-read...


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What would cause a Cessna 182T G1000 to show red X's on Manifold, Fuel, M & S Batt Amps?
Our 2006 Cessna 182T G1000 suddenly started showing the following X's on Manifold, Fuel, and M/S Batt Amps. You can see in the second photo, that the batteries are charging when the plane is started. No other issues it seems, and of course it's grounded till we figure it out.
I would think those 3 systems are completely independent of one another, except tied together by the GEA 71 unit. We checked terminals, connections, swapped PFD/MFD screens. The system check seems to suggest the GEA 71 checks out OK, but those 3 systems obviously are failing.
Any other ideas of what to look at next, aside from trying to put in a new GEA 71 and hope that solves it? Don't want to just throw new components at it though.

It's pretty simple.  If you want to avoid swapping out the GEA71 with a known good unit to see what happens, the way an airline would normally do it when they have spares on hand, you have to go digging around the old fashioned way first, and figure out which pins on P701 and P702 are being used for those input/outputs, and start checking back through the wiring to the source units with a DMM.
You need a wiring diagram that shows the wiring between the GEA 71 and the source hardware.
You aren't going to have those components all stop providing signals independently at the same time, so clearly there is either:

The failed source component wires all pass through the same terminal block or bulkhead connector and there is open connection for all those circuits.
Those inputs are processed by particular common circuit board inside the GEA 71 and that board has gone south, although you would normally expect that to show up during the unit's diagnostic or startup BITE.

So if it passes all BITE tests, you simply work back through the wiring looking for a common failure point, and if you can't find anything amiss, well maybe the BITE doesn't catch it, and it's time to swap out the box.

Thanks. We're going to be working on testing each component, and will report back what we found.


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How can I estimate the coefficient of lift for take off?
For the CLto (take off lift coefficient), is there some way to estimate this?
I was thinking of using CLmax but wouldn’t this be too big?  I was thinking maybe CL cruise?

For many airplanes the lift-off lift coefficient is determined by the maximum pitch angle before a tail strike occurs (minus some safety margin, of course). Before take-off, the speed which allows flight at this pitch angle is computed so rotation is not initiated too early.
If your configuration allows to be stalled with the achievable pitch angle during ground roll, taking off at this maximum lift coefficient is unsafe and most likely counter-productive. Normally, drag near stall is growing disproportionally with angle of attack, so rotating too early will create high drag which leaves less thrust for further acceleration. Also, any flow asymmetry between the left and right wing near stall might cause an unstable rolling motion which can ruin your day and the airplane with it.
For fast-accelerating airplanes a safety factor of 1.1 for lift-off speed can be considered (which means lift-off occurs at 82.6% of the maximum lift coefficient) and for more conventional types this could be increased to 1.2 (so the lift coefficient is only 70% of its maximum). Details depend on the shape of the drag polar and the stall behavior the particular design.
The lift coefficient for cruise will most likely be too low to allow for a reasonably short take-off distance, unless we speak of a motor glider.

"Normal takeoff attitude for the B737-800 is between 8 and 10 degrees.  This provides 20 inches of tail clearance at flaps 1 and 5.  Tail contact will occur at 11 degrees of pitch (if the aircraft is still on or close to the ground)." While the takeoff pitch angle is (by my standard) dangerously close to tail strike, it feels like the flight manual is very confident that the airplane is off the ground before rotation completes (which takes 4-5 seconds).

I think using tail strike pitch angle (or takeoff rotation angle) as an estimation of takeoff threshold AoA could be quite inaccurate, because liftoff is likely to happen before target pitch is reached, after which the AoA won't raise to target pitch either because the speed vector will also pitch up and eventually catches up with pitch angle.

@user3528438 If the B757 flies already at 10° pitch, it will not reach those 11° while still on the ground. Thank you for confirming my answer. And when liftoff happens before target pitch is reached, you rotated too late. Physics cannot be cheated!

Using the lift equation:

L = W = $\rho$ × Area × coefficient × V$^2$

For a safety margin of V rotation of 1.1 stall speed and a given planform we can simplify to:
Lift = k × Lift coefficient$_{max}$ × V$_{stall}$$^2$ =
k × Lift coefficient$_{rotation}$ × V$_{rotation}$$^2$
plugging in 1.1 rotation speed we have:
1.21 stall speed = Lift coefficient$_{max}$/Lift coefficient$_{rotation}$
Rearranging to Lift coefficient$_{rotation}$ = Lift coefficient$_{max}$/1.21
or 83% of Lift coefficient$_{max}$
Since Angle of Attack is (generally) linear to coefficient of lift,
AoA will also be 83% of critical stall value.
For a Cessna 172 cruise speed is roughly 2x stall speed:
Lift = k$_{172}$ × Lift coefficient$_{cruise}$ × V$_{cruise}$$^2$ => Lift coefficient$_{max}$/4 = lift coefficient$_{cruise}$ or 25%, but one would have to reach cruise speed to generate sufficient lift before taking off!
A safer medium might be 1.2 V$_{stall}$, yielding a Lift coefficient$_{rotation}$ (and AoA) of 70% of C$_{l max}$.
In reality, the 172 rotates at 1.1 V$_{stall}$ (55 knots$_{clean}$), but is accelerating.  It will lift off the runway at a higher airspeed.


text:
In Spain, can I land and take off from my own property?
I have a property in Spain with a big field; is it legal to take off from and land on your own property in a light aircraft?

I edited your question to fix spelling and add a couple relevant tags.  Welcome to Aviation Stack Exchange!  If you haven't done so, please read through the [FAQ] and take the [tour] to learn more about how Stack Exchange works.

There's going to be no "general" case ... imagine your land is under the end of the main runway at "pick your favourite airport".  And then there will be exclusion zones, etc.

It depends on your local and national regulations.  In some jurisdictions, local municipalities can regulate use of land in that way, and in others they can't.  So first check with Spanish national regulations (if separate from EASA) on what constitutes an aerodrome or airport to see if there are any restrictions on use of an unlicensed piece of land as an aerodrome (probably not), then check with local municipal regulations or zoning or use bylaws that may restrict use of your land as an aerodrome (a definite possibility).

No, unless you present a project of the airfield and get it approved by the municipal and aviation authorities. Not impossible, but very slow. Think in two or three years of negotiations before you can even start construction...

Could you cite the rule that makes this the case?

No, and not only because it's not just one rule. Besides, Spain is a very diverse and de-centralized country, so that different places have different rules.

If Spain is so diverse and decentralized, how are you sure that this is the true answer for all of Spain?


text:
Pilot without medical needs me to fly with him. Can I log?
I have spent most of my career in the military and just started flying for a civilian flight school, so I am still trying to learn some of the gray areas of the civilian world.
I am a CFI/CFII. An aircraft broke down and the maintainer needed to fly over. He needed me to fly with him (in a Cessna 172) since he did not have a medical. I was there making sure the flight went ok but he was the sole manipulator of the controls.
I assume for the flight, I am "acting PIC." Can I log that as PIC even though I did not touch the controls? It would not have to be dual received/given since I was not instructing, just me being there for safety?
It was only 0.4, it's not that I care about losing out on that 0.4, I am just curious and really like my logbook to be as accurate as it can be.
Thanks!

Look closer at 61.51(e)(1)(iii). Did your aircraft or operation require two pilots?

The aircraft doesn't (C-172.) But he did need another pilot, in this case me, since he was not medically qualified. I've looked in the regs and did a lot of googling, I can't seem to find the answer. I would've thought this situation isn't super out of the ordinary, maybe I'm missing something obvious?

I would say that for the purposes of the flight he was your student, you were PIC legally responsible for the flight, and you would log it all as PIC just as with any instructional flight.

You were the only qualified pilot on the airplane.

Short answer: no, you can't log it as PIC time. Remember that acting as PIC and logging PIC time are two completely different things.

I'm assuming here that the flight was VFR under part 91. The rules for logging PIC time are in 14 CFR 61.51(e). Simplified - and ignoring the ATP and student scenarios - the times you can log PIC are:

Sole manipulator of the controls, if you're rated for the aircraft
Sole occupant of the aircraft
Acting PIC when more than one pilot is required by the aircraft's type certificate or the regulations for the operation
Providing instruction

#1 doesn't apply because you didn't touch the controls. Neither does #2 because there were two people on board. #3 doesn't apply either: even though you were acting PIC, the C172's type certificate doesn't require more than one crewmember, and there's no regulation that requires more than one crewmember for VFR under part 91. As for #4, you said that you didn't provide instruction (which would include endorsing the other pilot's logbook to record the training).
I suspect you're getting a little distracted by the idea that you were there as a "safety pilot" but in the regulatory sense of 91.109(c)(1) that only applies to simulated instrument flight.
Assuming that the pilot who actually flew the C172 was rated to do so then they can log the time under reason #1 above. The regulations don't require you to have a medical to log PIC time.

Is the conclusion, then, that there can be flights for which no one can log the time? For instance, if a non-CFI pilot allows a non-pilot passenger to fly an entire flight (e.g. a parent helping a child to learn on days when the child's instructor isn't available), then the flight is therefore unloggable?

@KennSebesta Yes, that's correct. The parent is acting PIC but can't log PIC time for the reasons I explained, and the child can't log it either because they aren't rated for the aircraft (even if they hold a student pilot certificate, students can only log PIC time when solo). As I mentioned, the key point is that acting as PIC and logging PIC time are two different things; acting as PIC doesn't automatically let you log PIC time.

I understand/accept your reasoning but it strikes me as odd to have a plane with unlogged flights. Maybe that's perfectly fine and appropriate, but in aviation where we have log books for everything down to the propeller, it's just so weird to think that there can be this legally required disconnect. Would the same apply to T/Os and LDGs? Could I legally have fewer T/Os than LDGs and vice versa?

@KennSebesta I'm not sure where the disconnect is. Many private pilots don't log their flight time at all, because there's no need to and it doesn't benefit them in any way. Think of someone who's retired and flies their Cub for lunch nearby a few times a week. The FAA doesn't require pilots to log anything at all unless it's to get a new certificate or rating, or to document currency.


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How much does glider insurance typically cost in the UK?
I (and my colleague from the club) plan to buy K8B glider. Does anyone know how much the insurance can be? We are both "just solo" pilots in our 50s. All insurance companies require details we do not want to provide as we did not purchase the glider yet. I am just curious if it is a few hundred or a few grand a year?

Just now,  one of the insurance companies has called me back.
They told me that the insurance premium will be between £290 - £790 depending on the cover details and the equipment insured.

Per year? Does it not depend on the amount of flying?

@Zeus That is what a guy from the insurance company told me.


text:
How does the advancing wing in a flat spin create nose thrust?
Peter's answer to this question leads me to another question.

How does the advancing wing in a flat spin create nose thrust?
I imagine this like some object spinning in a liquid.  I would think that the motion of each wing would only create drag that acts as torque opposing the motion of the spin in the geographic plane of the spin that would tend to make the spin slow down rather than be maintained.

A very good question, considering so many people do not draw the thrust vector for gliders.
The wing generates thrust by converting energy from falling to forward motion.  From scratch building gliders (and reading) one comes to realize that a falling parachute (straight down) only uses drag to reach a constant rate of descent.
A glider, with its center or gravity off set from its center of pressure (including tail!!!) will start to move sideways (glidogenesis) and generate lift with its wing.  This motion, known as "thrust" in this example, uses altitude as fuel and falling as its engine.
Notice both wings may be "thrusting" as it corkscrews down, but the outside wing, with its lower AOA, generates more lift/thrust and much less drag, sustaining the spin.
Once yawing is stopped, the wings equalize, and can be unstalled by lowering angle of attack.

I see. So it would be better to envision the airplane like a falling rotor blade being struck from the bottom and pushed forward since the plane is tilted nose-down.

From the auto rotation point of view, yes.  You can make a business envelope glide by offsetting drag from falling and center of gravity, and nose down also makes it easier to recover.  It is powered flight (gravity).

I think Peter's diagram explains it well, I've repeated it here:

You can see that for the advancing wing (right side of the image) the vector addition of $V_\infty$ and $\omega_z \times y$ gives a resultant force R, that has a component pointing forward. This is forward thrust and increases the spin.


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How do the air conditioning packs work for the CRJ 700/900?
Please provide details on how the packs on the CRJ 700/900 work. How does the system use air from the packs to make cold air for the aircraft?

I am pretty sure it works in general like all other A/C systems.

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What do you need to do to bring a 737 Max back in service after 6 months in storage?
What kind of maintenance has to be performed on an aircraft if it has been stored for 6 months?
For example, do fluids need to be changed, have the tires become flat, etc.

No airplane experience but I've gotten generators, cars, weed whackers, etc. all going after sitting for extended time.  In all of the cases the biggest thing was stale/old fuel in the system.  Doing nice things like clogging the jets on carbs, breaking down gaskets, etc.  Brake lines swell, etc.  Tires get flat spots.  Sitting is the worst thing that can happen for something mechanical like that.  Powering up the engines, getting 'em up to "working RPMs" and taxing up and down the run-way and then parking again on a weekly basis would probably be better than static stagnant storage.

@ivanivan Aircraft are much more complex machines than your day to day vehicle/tool. Not just because there are many more systems and subsystems aboard, but the overall tolerances for failure and wear and whatnot are waaaay tighter than you can believe, all things considered. If an aircraft is left to sit for 6 months, there are many more procedures and check lists to go over than just occasionally starting the systems and wheeling it around here and there.

I really like the question as it mays also apply to other airplanes

Bringing any plane back into service will involve more or less the same things but vary in scope and scale depending on the size of the aircraft. It looks like Southwest is moving their 737's to their long term storage facility. Since its in the desert they may wrap/plug the engine inlets to prevent any debris/dust from getting in. This will need to be removed before the aircraft are brought back into service.
Aside from consumables like fuel, oil and hydraulic fluid, etc which are checked before every flight they will need to check all calendar-related maintenance items. Most aircraft maintenance items are tied to hours or cycles which don't really matter when the airplane sits, but some things are tied to the calendar and a sitting plane may lapse on checks and updates it needs. Off the cuff a plane sitting for a while may lapse or need replacement on:

Bi-annual Transponder Checks
Navigation/Terrain Database updates
Fire extinguishers past check/expiration
Expired Oxygen generation canisters
Expired slides/lifejackets (mostly the chemical inflator systems)
Batteries may need tending/replacement if run dead (or allowed to be run dead)

From a non-hardware standpoint, crew currency or type proficiency due to lack of flying hours on the airframe may land a chunk of pilots and FO's in the sim before the planes come back into service.
Jet fuel can grow bacteria in the tanks and a tank flush/purge may be done on aircraft that have been sitting for a long time depending on what they find when they get to the aircraft.

-----EDIT------
Since more information has been released since the original answer was posted. It now appears most airlines are expecting 100 - 150 hours of work to bring the current resting 737's back in to service after the software update gets approved. This only applies to the mechanical aspect, pilot training is still under debate:

each aircraft will likely require between 100 and 150 hours of preparation before flying
....
The preparations were discussed at a meeting between Boeing and MAX
  customers in Miami earlier this week, and include a list of items
  ranging from fluid changes and engine checks to uploading new 737 MAX
  software. The estimated time frame does not include pilot training,
  they said.

Interesting choice to call a test for microbial contamination a 'pregnancy-style test'...  Congratulations, you have fungus in your wings...

@Sanchises It's a boy!!!

Or a yeast infection.

Considering it takes about 80 hours to put a plane into storage (from the Reuters article linked above), 100-150 to get it out does seem to be reasonable.

Just clarifying - those are person-hours?  So a team of 20 could do approximately one plane a day.

In addition to the modifications that will be listed by the FAA specifically for the 737max, generally speaking before bringing back Into service any aircraft, you need to know what has been done to store the aircraft: the engines have been removed or not? the hydraulic fluid  has been exchanged with special storage fluid, or the hydraulics have  been periodically recycled using external powering? The list of possibilities is long, therefore to bring back into service you must know ahead what has been done before storage and you must know the storage conditions.


text:
Has a passenger's consumer electronic device ever actually caused a crash?
Most of us are familiar with the standard warnings and regulations against passengers using electronic devices (at all during takeoff and landing, and in some cases throughout the flight). This is based on a concern that sensitive avionics or aircraft radio equipment could be affected by interference from devices being used by passengers.
Has a commercial flight ever crashed as a direct result of a passenger using an ordinary consumer-grade electronic device during the flight? More specifically, has there ever been a crash investigation where the conclusion was e.g. that the plane wouldn't have crashed if the passenger in seat 5-F had turned off his Game Boy before takeoff?
To be clear, I'm not asking about why there are electronic device regulations or safety guidelines. I'm asking if these guidelines are purely theoretical (we're pretty sure that using them could cause a crash, even though it's never happened) or if there have been actual crashes due to device usage.
Also to be clear, I'm asking only about ordinary, consumer grade electronics (e.g. phones, laptops, tablets, beepers, walkie-talkies, video game consoles, etc.), rather than military or industrial equipment.

A friend, a retired Capt for AA, and also a ham radio guy, has told several stories about NAV interference in the VOR days from consumer electronics.  In each case he was able to establish that the NAV was unaffected with the device off, and was affected with the device on.  Only with VOR, not GPS, LOC, ILS, etc.  Given his degree of RF savvy, I give his accounts high credibility.

The answer is likely no, but that's not the reason for the regulation The reason is potential interference with aircraft systems.

Related: Personal Electronic Devices and Their Interference With Aircraft Systems, a bit old (2001). Crashes: Never; Incidents: Only suspicion, not well supported: "there is one category of the database that provides supporting credibility to these events--pilot flight hours. The total mean flight time of 10,790 hours from Table 1 indicates that pilots reporting PED events are very experienced." One thing to consider: If 300 persons emit with their 2W cell phones, that's 600W. Unlikely, but...

I would also like to add a sub question to this: Is it possible that it's a question of number of devices? E.g. 2 out of 300 pax using cellphones vs 298 out of 300 pax using cellphones.

@Mat sure, I'd allow that, but only if such was found by crash investigators or otherwise demonstrated with evidence. I'm not looking for idle speculation ("Oh yeah, MH370 went down as part of the epidemic of electronic device use, please turn yours off!!11!1one").

It's a real problem in communications. Phones are to be turned off in the Center control room, but someone always forgets. When a phone is on vibrate, and goes off while you're transmitting, your sidetone sounds like heterodyne if the phone is close, and like someone tapping, if farther away. It also "goes out" to the pilots.
The center has a charging kiosk (with locks) outside the main control room doors, and controllers are encouraged to leave there phones there, or in their lockers.

There has not been an actual crash to my knowledge, but I personally have had passenger cell phones interfere with my comm radios several. Once to the point where we couldn't hear anything but the interference noise and I had to call the flight attendants to tell them to find the phone and turn it off immediately. The interference stopped soon after that.


text:
Can a seaplane make a long distance sea voyage on the sea?
In general, if a seaplane can fly X distance, can it travel that distance on the surface of the water as if it was a boat or ship? In other words, this would be equivalent to taxiing all the way to a destination. E.g. if a seaplane has the range to fly from New York to Amsterdam, could a pilot simply choose to taxi all the way across the Atlantic Ocean and casually slip into the harbor?
If this is not generally possible, what rules define what sort of sea journeys a seaplane could undertake? Is there a coefficient of inefficiency (e.g. if a seaplane can fly X miles, it can taxi X/2 miles due to increased drag from the water)? Are the vast majority of seaplanes not set up to handle the stresses of a long-distance sea trip, making them likely to result in a shipwreck (er, seaplanewreck)?
This is not the same question as Is it possible to sail a seaplane? . That question is asking about sailing a seaplane using sails without any engines. I am asking about "sailing" a seaplane using the engines and whatever other tools are available for taxiing.
This question is about the physics and engineering issues - not about licensing. I am fully aware that taxiing a seaplane across a body of water might make the trip fall under boat regulations and require a sea captain's license, but I'm not asking about that.

Water is something like 8 times more viscous than air. It wouldn't have enough fuel to make the same trip floating as it would flying.

Not in the water, but just above it would be the best way.

@FreeMan It would if it would travel 64 times as slow.

Another factor here is that the engines (and probably the props) aren't efficient at low speeds, so even neglecting the higher water friction, you'd need a lot more fuel to travel a given distance.

Not very well. If we look back to the 1930's, the age of the large flying boat airliners, they were designed to take off and land in minimal sea states - no more than 3 foot swells. Typical usage would be in large rivers or protected bays, and the flying boat was used because long runways weren't available. Nor did those airliners tend to stay on the water for long, usually they were brought onto land with beaching gear if they weren't in service.
The problem is - an aircraft on water is highly susceptible to changes in wind - it is, after all designed specifically to use flowing wind to remain airborne, so gusts of wind while on the water will have a stronger effect on an aircraft than they would on a ship. As well, the aircraft's hull in the water tends to anchor it, subjecting the airframe to stresses from wind in varying directions, that it wouldn't encounter while flying.
Amphibious aircraft and the pure flying boats, left on the open ocean, tend to become damaged to the point of becoming non-airworthy.
Couple of examples:
While on lifeguard duty off of Truk during WW2 and landing to pick up a shot down pilot, Lt John Dowdle's OSU-2 Kingfisher had overturned due to a gust of wind. Lt John Burns also lost his Kingfisher due to the hull springing leaks from being pounded by waves, while he rescued Dowdle and the downed pilots. Both Kingfishers were lost, but their crews plus the airmen they had rescued were saved by submarine USS Tang, in an epic lifeguard mission that saw Tang pick up 22 aircrew.
The Bermuda Sky Queen, a Boeing 314 formerly known as the Pan American Capetown Clipper, ran short of fuel while flying from Foynes, Ireland to the US. It did survive for over 24 hours on the ocean before the 62 passengers and crew could be rescued, but after that much time on the open ocean, the plane was coming apart from the pounding of the waves (the tail section was starting to come loose) plus damage from colliding with the Coast Guard cutter picking up the people. As a result, the Bermuda Sky Queen, the last airworthy 314, had to be scuttled.

I once learned how to drive a motor boat and I agree with this answer. Going from the harbor area to some outer islands then further out to the Pacific, the difference in swell is huge. The largest practical obstacle is not efficiency - a seaplane would likely disintegrate / sink before it runs out of fuel.

@kevin I own a motor boat and take it out on the Indian Ocean for fishing. It is clear to every boat driver that small boats are not the best.for crossing oceans. Sea state is an issue, but only part of the question.

@Koyovis the main question is can a seaplane make a long distance sea voyage, this answer provides a valid reason why it couldn’t, looking from a structural point of view. It’s a different view from your answer but equally valid (possibly more valid).

In the water: it could reach its fiinal destination, if:

it would travel very slowly, as slowly as a boat;
it could use water for creating thrust instead of air;
the sea state would be calm.

Image source
The main drag resistance on a boat is the surface wave (wake) drag, much higher than the submerged incompressible drag deeper down. Wake drag increases dramatically at a certain speed-to-length ratio, like hitting a brick speed wall. That is why hydrofoils work: they lift the vessel out of the water and eliminate wake drag.
The other bit about range efficiency is in the propulsion. Highest efficieny is obtained by accelerating a high mass stream to a lower velocity, and water has a density that is 1000 times that of air. Let that ships' propeller lazily turn about to propel the long supertanker to a slow speed, and there is not much that can beat that efficiency. Perhaps a road train running at 5 km/h. The flying boat propelling air cannot do it.
Speed is the governing factor in travel efficiency. Aircraft only exist because people want to be somewhere now.

Are the vast majority of seaplanes not set up to handle the stresses of a long-distance sea trip..?

Flying boats need to fly and are not designed for handling violent seas. They sit relatively high in the water and would topple over if they had no outrigger floats - picture that set-up in a storm and it is easy to see a wing break off or the craft capsizing.

From here looks like an airboat (propeller driven boat) is not very efficient mean of transportation, using somewhat 0,1 l of fuel per minute (so 6 l per hour) when moving at roughly 2 knots (so 3,7 km/h), so only 0.6 km per liter. Assuming we have somewhat 160 l fuel tank, this is enough for about 100 km journey.
If it the "airboat" is actually a seaplane, probably would rather fly. It may be even less efficient as has not been designed for taxiing on water over hundreds of km.

Is it possible using the engines/propellers, at taxi speed, if you have a mean to refuel.
As the fuel consumption will be about the same per unit of time, the cruise speed vs taxi speed ratio is meaningfuel, you will need to refuel 15 or 20 times.
You don't use anymore lift, and the craft is not anymore an aircraft. This possibility would be silly. The solution is to increase speed by lifting the hull with hydrofoils.

Lisa taking off with hydrofoils, source
The actual lifting surfaces are now the hydrofoils (air/hydro foils), they create lift from the water flow. Since the craft is only used over water, it can be optimized, and the wings can be shortened.

Source: Wikipedia

As a hydrofoil craft gains speed, the hydrofoils lift the boat's hull
  out of the water, decreasing drag and allowing greater speeds.

I think the question is about whether one can use an existing seaplane to cross an ocean, not whether it is feasible to design a craft specifically to cross the ocean.

@kevin: I clarified.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
What's the difference between overrun and overshoot landings?
Is there a difference between overrun and overshoot landings or are they exactly the same, talking about a runway excursion?

Overrunning or overshooting a runway aren't in fact rynway excursions. A runway excursion means entering a runway without clearance and causing danger to other traffic.

@Sami: No, that's a runway incursion.

@Sean you're right. My bad. Second language problems

They are more or less the same and are often used interchangeably. FAA AC 91-79 Runway Overrun Prevention defines runway overrun as:

A runway overrun event is a departure of the aircraft from the end of the intended landing runway surface.

It means that the aircraft has touched down on the runway before going off it. Overshoot is used in the same sense (note that overrun/overshoot occurs both in TO/landing).
However, there is one case where they are used differently. Overshooting the runway also can mean that the aircraft has touched beyond the end of the runway i.e. missed the runway entirely.

Note that in Canada overshoot means go-around

Same in the UK.

In the US Navy an overshoot meant flying through extended runway centerline during the approach turn, and having to come back for line up.  (vs an angling approach)


text:
How to read the propeller range chart for DHC-6 Twin Otter?
I understand constant speed propellers and beta range. However, I'm not completely sure I'm extracting everything from this chart that appears in a manual for a flight sim model of the Twin Otter.

Is this format common for describing propeller range in aviation?
Can someone describe the information exposed by the chart in words?

Source - Aerosoft manual

I'm curious why there are 2 zero degree markings on the chart.

@Johnny Dead zone, maybe? The light is a warning that you're entering beta range so you don't inadvertently go there in flight, I presume.

I think part of my confusion comes from the power and propeller lever blocks on the left side. It seems like the power lever must be full forward, then advance the propeller lever, but that's not how it actually works...

@Johnny -- I wonder if one of them is a typo, meant to be +10 degrees?

Most of high performance propeller aircraft use constant speed propellers, with variable pitch. The speed remains constant over (most of) the flight regime, while the propeller pitch is varied according to the requirement (by the Constant speed unit or CSU).
In case the propeller is stopped in flight (for example, due to engine failure), the propeller pitch is increased so that the propeller is basically parallel to the airflow, so as to minimise the drag. This marks the extreme value of feather angle 87$^{\circ}$ shown in the image.

Image from 12charlie.com
The propeller range used for forward  flight is usually called the $\alpha$ range, where the blade pitch has moderate values with constant speed, which is set by the propeller lever. Obviously, this speed is not constant over the entire range between engine start and shutdown. Usually around the idle, the rpm drops (this varies according to the design) and this gives the constant speed range limit.
Now, during landing, the aircraft speed is reduced usually using brakes etc. However, it is possible to do so using propeller itself- basically by turning it backwards. The $\beta$ range refers to the region where the propeller is producing zero or negative thrust.
From this article:

On PT6 powered aircrafts, the Bêta range is controlled by the PCL ( Power Control Lever ) which also directly control blade angle. Yeah, the governor is overriden and by moving the PCL past the idle gate ( you usually have to lift the PCL against that gate ) you directly schedule blade angle in correlation with Ng ( Gaz generator ) speed.

In the beta range, the aircraft can be slowed down by making the propeller pitch flat (0$^{\circ} 30'$ here) and creating drag. The reverse thrust starts after this, with the full reverse trust coming in at the limit -15$^{\circ}$.
I think there is some typo in the figure. From this answer(cleaned up):

Beta range in the Twin Otter is from 11° to -15°(reverse) propeller blade angle.
The Beta range lights will be lit when Your in the range from 9°to -15° propeller blade angle.

When in the beta range, what is the expected position of the propeller lever? I guess this bit of information isn't represented in the diagram, and I assume it's full RPM... Also, is this a typical way to communicate this information in thee aviation world, is this typical of de Havilland, or of my flight sim manual?


text:
What conditions cause the gear-up warning to sound on a Bombardier DASH-8?
As the title says, what conditions cause the gear-up warning to sound on a Bombardier DASH-8?

It's DHC Dash-8 now :)

According to smartcockpit.com, warning is given when the gear has not been extended and the aircraft is in the landing configuration. Specifically,

• All landing gear not down and locked, flaps selected to 15 or 35$^{\circ}$, and both power levers at any position less than 50% torque.
• Landing gear not down and locked, one or both power levers at or near FLT IDLE and airspeed below 130 kts.

The warning also sounds when the aircraft landing gear is up and terrain clearance is inadequate during the cruise and approach phase
of flight.

Image from Dash8-200/300 - Warning Systems from smartcockpit.com

How is the aircraft "in the landing configuration" if the gear isn't down???

That's what the aircraft is trying to figure out with the warning sound :)

To further elaborate on aeroalias' answer, "terrain clearance inadequate" is determined by the radar altimeter.  If the radar altimeter returns an altitude lower than the threshold and the gear is not extended, the warning will sound.


text:
Why do double-ended ILS runway share the same frequency from both sides?
Let's find a runway where there are localizer-related procedures at both ends, e.g.

KMRY: 110.7 for both ILS RWY 10R and LOC/DME RWY 28L
KLAX: 108.5 for both ILS OR LOC RWY 06L and ILS OR LOC RWY 24R

However, this doesn't align with the ILS back course concept. When I fly one procedure, how can I can tell that I am not on the back course of the other? Getting down to instrument interpretation, for an HSI, how to set the front course? In a traditional VOR, is it positive or negative sensing?
POST EDIT:
Tried in a SIM, it seems that when well above the slope, it shows back course. When I get lower, it shows front course.

I think every ILS in Indianapolis (KIND) has the same frequency for each runway.  Three runways, thre frequencies, six ILS approaches.  Tower turns off the ILS's that are not in use for the opposite runways.

They are definitely not the same transmitter. Note the shaded half is the right one for the approach course on both charts. They are indeed both normal sensing.

Listen to the identifier... That will tell you if you have the 10R I-MRY or the 28L I-MTB ils.  Some runways use the same frequency like this, while others use different frequencies for opposite direction ILS's.  The identifier will always be unique, though.
The 10R transmitter doesn't operate at the same time as the 28L transmitter. One transmits "this" ils beam in "this" direction using "this" ident on whatever frequency; the other transmits a different ils beam in a different direction using a different ident; the fact that it is or is not the same frequency is incidental.
Usually, the two reciprocal ILS's to the same length of pavement use the same frequency, with one switched off when the other is switched on.  MRY in the OP is a good example of this, and most other airports I looked at have the same behavior, but this is not universal.  Reno, NV -- KRNO -- is an example where the opposite ILS's are not on the same frequency: 16R ils is 110.9, but 34L ils is 109.9. Same pavement, different frequencies.  Also, Detroit, MI -- KDTW -- appears to have two full ILS's set up for runway 4R/22L, with one pair on 111.75, and the other on 111.95.
One other note: the various flight simulator programs may or may not accurately model this behavior.  Some may simply have every ILS "on" at the same time, so if you're west of "this" airport, you'll receive the ILS for landing east, and vice versa.  Works well for a flight simulator program, but that's not how the real world works.  It takes a call to tower to have the one ILS turned off and the other turned on, and it isn't something they prefer to do repeatedly or on short notice.  In the sim, it's convenient to fly approaches back & forth to 16C then 34C then 16C and so on, but even if traffic permitted, in the real world they'd probably prefer you flew your ILS's to one runway, and use something that doesn't need the ILS or the LOC when landing in the opposite direction.

I am wondering how is it possible to broadcast two IDs from the same frequency?

First, both ILS may not be in use at the same time (usually, the runway is in use from one side only). But even if both transmitters are used at the same time, they are emitting in opposite directions. In final, your plane will only receive the signal from the transmitter pointing at you. ILS antennas are directional, not omni-directional.


text:
Why don't planes use pulsejets?
According to my research, a pulsejet's thrust grows as the airspeed increases. Most pulsejets require no moving parts which makes it easier to manufacture them. They are also very lightweight which can have some great effects on aircraft.
They are very slow at zero airspeed but very fast at high airspeeds. Therefore you could simply have a normal jet engine to power the take off and then switch after take off to the pulsejets.
Here is an example where I've labelled a DC-10 of what a pulsejet with jet engine plane could look like:

The plane would taxi onto the runway using the jet engine as power, it'd then line up on the runway. It'd then power its jet engine and take off. Shortly after take off it would switch to the pulse engines and optionally turn the jet engine off.
So why don't planes use pulsejets? Surely they would allow planes to go faster and in turn allow them to reach destinations quicker. Also the pulsejets are easier and cheaper to produce than jet engines and there lighter than jet engines.

It's been tried, didn't work.

@DeerHunter It actually did work. They use them for target drone aircrafts and other aircrafts.

"They are very slow at zero airspeed but very fast at high airspeeds" I guess that is true for everything that moves.

@PeterKämpf I meant that the produce more thrust at high airspeeds than zero airspeed.

@Hyden: I understand that, but I just couldn't resist ;-)

Would it not make more sense, if you wanted to use a pulsejet with all its drawbacks, to reverse the engine configuration?  2 conventional engines, and 1 pulse?  That way, plenty of power would be available for takeoff/landing, and low-power cruise would be covered, too.  Also, with a failure, there would always be at least 1 conventional engine for landing.

@Jeffiekins Perhaps, I was just suggesting what a pulsejet could look like.

@Jeffiekins: Cruise at high altitude (= low density) is far more economical, so cruise is also a high-power affair.

Better still improve pulse jet technology so that it produces more thrust at low and high speed without adding weight or actually further reducing their weight

2 reasons really:

Airplanes need most of their thrust right when their airspeed is low. (to get more airspeed at takeoff and go-around).
The maximum airspeed in cruise is not dictated by how much thrust the engines can throw out. Instead it's dictated by the aerodynamic forces on the airframe. They cruise right below the speed of sound where the rules of aerodynamics change drastically. Going too fast would rip an airplane apart.

I'd add another: noise and vibrations. Pulse jets, or the more promising pulse detonation engine (PDE) are not exactly renowned for their quietness.

In your proposed design an engine failure at low speed would result in essentially no thrust.  So, additionally you would need to carry a second gas turbine engine for redundancy.
Then, during cruise you would be carrying around these two extra engines that are not being used, so they are just dead weight.  Overall, airliner engine selection is driven largely by fuel efficiency.  Wikipedia reports that pulse jets have poor specific fuel consumption.
Also, pulse jets tend to be very loud, which can be a noise problem.

Activate the jet engine or leave it on? Ignoring the fact that an airplane shouldn't really be flying at low speed at any point except take-off and landing at which points it will be using the jet engine. Unless of course the plane is doing loop-de-loops.

@Hyden What Adam is saying is that if the turbine engine failed, you would have nearly no thrust during takeoff, which is exactly the time when it's most critical to have as much thrust as possible. You'd have to have at least 2 turbine engines in case one of them failed during or shortly after takeoff and the remaining engine(s) would have to be large enough to have enough thrust to complete the takeoff and climb out in the event of such a failure. This is already a legal requirement for transport category aircraft.

Also, not very long term stable, especially the valved type - the V1 engine was probably too worn to reuse even if it wasn't used in a way that INTENDED burning and ripping it to shreds upon reaching destination....

@Hyden no thrust at TO/approach = crash = crash into city = mass death

There are multiple reasons for pulsejets not being used in aircraft (or manned aircraft generally).

They are quite loud. In fact most of the pulsejet aircraft are (in)famous for their noise. One of the reasons for the US Army to reject the pulsejet powered XH-26 was the unacceptable noise.

"XH-26 Jet Jeep in flight" by San Diego Air & Space Museum Archives - http://www.flickr.com/photos/sdasmarchives/4561399305/in/photostream/. Licensed under Public Domain via Commons.

The intermittent (combustion) operation means that vibration is a serious problem. For aircraft engines this causes serious problem; especially for commercial airlines, such vibration levels would be unacceptable.
Though pulsejets can be operated in low (and even zero) airspeeds, it uses the dynamic pressure of air for compression. Hence the engine will be producing little thrust in the most critical phases of operation (takeoff and climb).
Pulsejets' combustion chamber operates at high temperature and requires heat resistant materials. Air cooling can improve this, but will complicate the engine.
There are other things here- having a dead engine (assuming that the turbofan is turned off at some point) simply increases drag and weight penalty.


text:
In a spin, are both wings stalled?
I missed a test question which asked that if an airplane was spinning to the left which wing was stalled.  The supposed correct answer was that both wings are stalled (I had answered that the left wing only was stalled).  However after looking at the this article on Wikipedia it seems to indicate that only one wing needs to be stalled to spin:

In a normal spin, the wing on the inside of the turn stalls while the outside wing remains flying. It is possible for both wings to stall, but the angle of attack of each wing, and consequently its lift and drag, are different. Either situation causes the aircraft to autorotate toward the stalled wing due to its higher drag and loss of lift.

So my question is was it fair for me to have missed that test question since according to Wikipedia a spin can occur with only one wing stalled?

I would like to note, since the question is tagged with "faa-knowledge-test", the answer is "yes" because the FAA is like the trivia guy at the Mexican restaurant on Saturday night... whether they're right or they're wrong, they're always right; that's just how the game is played.

I've been exploring spins extensively recently with model airplanes, and that exploration has been driving home the idea that there are many different kinds of spins.  In a plane with lots of elevator throw, it's very common for upright spins with full aft elevator to be flattish, with an almost level pitch attitude, and a low rotation rate.  Decreasing the up elevator deflection-- including sometimes all the way to neutral or sometimes adding some down elevator deflection-- typically causes the plane to pitch to a much more nose-down pitch attitude, with the spin rate greatly increasing

-- perhaps because the outside wing is now mostly or fully un-stalled, while it was mostly stalled when the elevator was full up?  On the other hand, in another case the plane could be induced to enter a flat spin with a very high rotation rate, yet at the same CG position, it could not be induced to enter a more normal (more nose-down) spin.  So-- when the subject is spins, every case is different, and beware of cookie-cutter one-size-fits-all answers in textbooks!

As a learning pilot, erm, living human being, I wouldn't trust wikipedia to be the authoritative answer for anything, let alone a life-or-death aviation situation.

No, one wing has at least partially attached flow. How else would there be a rolling and yawing moment which keeps the spin movement alive?
During a spin the aircraft experiences a linear variation in angle of attack over span. The pitch attitude is between 40° and 60° nose-down, and the local angle of attack is 90° minus the pitch angle, which is between 50° and 30°, at the center wing. Move outward from there and the angle of attack increases on the retarding side and decreases on the advancing side.
As a consequence, the outer advancing wing will experience a moderate angle of attack which can even become negative at the tip. Therefore, a sizeable portion of that wing side has attached flow with high lift and low drag. On the other side the angle of attack grows to 90° and beyond, so the wing is fully separated and the aerodynamic force is normal to the wing surface. See below for a diagram of the flow direction: The dark blue vector is from the falling motion and the red vector is the product of the yawing motion $\omega_z$ times the wing station y. Together they combine to the green vector which produces a resulting aerodynamic force R:

On the left is the retarding wing and on the right the advancing wing. Note that the aerodynamic force is in line with the flow vector on the retarding wing with its fully separated flow while the aerodynamic force is normal to the flow vector due to the attached flow on the advancing wing. The difference in the local forces produces a yawing and rolling moment which balances with the damping forces. If there would not be such an asymmetry, the motion would die down quickly.
Even in a flat spin, where the pitch attitude is around 0° (resulting in 90° angle of attack at the center wing), the advancing side of a moderate to high aspect ratio wing produces some nose thrust from partially attached flow. How else would the aircraft keep spinnig? Low aspect ratio designs produce a propelling nose vortex on the forward fuselage which keeps the motion alive.

So this is kind of like a helicopter rotor retreating blade stall where the retreating side can stall due to higher angle of attack?

@DLH: In a way, yes. But not cyclic, which causes all kind of funny oscillations from hysteresis on helicopter blades.

The argument that at least one wing has partially attached flow is meaningless. The question is, is the wing as a whole exhibiting the behavior in the plot I used. Airplanes stall all the time with portions of the wing unstalled. That’s why we twist the wing aero or geometrically, to keep roll control “post stall”.

@MikeY:  I believe that you are referencing washout which means the root of the wing will stall before the tip and this is used to keep some roll stability in the stall.  However I believe the situation Peter presents here is that the tip stalls before the root.

"How else would there be a rolling and yawing moment which keeps the spin movement alive?"  Rhetorical questions are unbecoming, Peter!  :)

@PeterKämpf How can the widespread, commonly-accepted knowledge that "both wings are stalled in a spin" be a myth?  Is the aviation community really rotely spouting bad information in the same way as medical professionals who tell diabetics to eat whole grain bread and drink diet soda without second-guessing the standard advice passed down pilot to pilot or are there exceptions to this answer for non-flat spins in which both are indeed stalled?

@RyanMortensen: There is a lot of nonsense making the rounds in pilot circles. I have not encountered the "both wings are stalled" myth myself, but how false it is depends on what stalled means. Note that I (indirectly) say that the inner part of the advancing wing is stalled. But what counts is the outer part with the long lateral lever arm, and that is not stalled.

@PeterKämpf "How else would the aircraft keep spinning?" That depends what the pilot is doing with the rudder. A flat spin doesn't mean zero forward airspeed! But you are right, most of this is a debate about terminology, not physics. It may be true (for some definition of "stall" ) that both wings are stalled in a "straight and level" full stall, but if that stall evolves into a spin one wing then becomes unstalled. But from the pilot's point of view, so what? Carrying out the standard recovery procedures don't depend on knowing the answer to that sort of trivia question.

I think you can add that the wing on the inside of the spin will have a lower airspeed than the one on the outside, which will exacerbate things.

I think this answer is wrong. Stall has a clear definition. If you’re above stall AOA and there are uncommanded pitch roll and yaw movement, you are definitely stalled. It doesn’t matter if there are tiny bit of sporadic lift here and there on the wing. This is a aerodynamic wing of course there will be some lift in some section of the wing in any imaginable situation. But that’s not enough to call it not stalled.

@Kolom I think the answer is right. Your definition of stall concerns the whole airframe, my only concerns the local wing section. Looking at the whole airframe only is not helpful for understanding spins.

@PeterKämpf by that logic, aircrafts never stall. Because even when in full stall there is positive AOA on some tiny bit sections of the wing. And It still producing some lift. So, why aircrafts fell out of sky then? It’s like looking at the rough sea and saying but it’s a calm see because when you look this small tiny section there is no waves here, it’s flat. You just need to zoom in more.

@Kolom You are free to provide a better explanation of why airplanes spin using your ocean analogy. I'm looking forward to it!

Ocean analogy was not for explaining how aircrafts spin. It was for showing a faulty logic. I guess it was not clear enough. Answer by @MikeY is good enough for me. The original question is asking which wing stalls in a spin. So the answer should be both.

EDIT 9/14/22
Correct FAA answer: Yes, both are stalled.
Reality: It depends
For a steady state spin, the inward wing has to have less lift and more drag to sustain the combined roll and yaw motion. You can get there with the outer wing stalled or not stalled. Two youtube videos of spins, one with both wings stalled, the other without.
Both stalled

Outer not stalled

My Old Answer

I guess a nit-pick is on "what is stalled"? I adopt that you are at or beyond the point that an increase in AOA results in an increase in lift (critical AOA). That's the top of the blue curve in the plot below.
Also, a stalled wing does not mean every point on the wing has unattached flow. It means the wing is operating at an AOA where an increase in AOA results in a decrease in lift.
At low angles of attack (AOA) planes are naturally stable in roll. The downgoing wing sees a higher AOA which results in more lift, and a restoring force. The upgoing wings sees a lower AOA, and less lift, so it is stabilizing too.
At a high AOA, though, you are operating on the backside of the wing lift diagram. In the picture below, this would be at and beyond 20 degrees AOA.

Now, the upgoing wing sees more lift, which leads to positively reinforcing going up. Same but opposite on the downgoing. It sees less lift.
Also, the red line shows drag. That downgoing wing (on inside of the spin) sees a great increase in drag, which will lead to a yaw towards that wing, i.e., pro-spin.
So to get in the situation where a roll/yaw movement is positively reinforcing, you need to be in stalled AOA. You might start with just one wing, you'll get to both.

I think this is a good answer.  I've been reading up on spins.  While I think that maybe only one wing will stall during the departure phase of the spin, by the time the spin is in the developed stage both wings will be stalled.  I think the test question could have been better worded though.

@DLH I think this is a poor answer because it is wrong.

@PeterKämpf:  Oh man I wish you had answered sooner, I was persuaded.

In your answer, you ask, "What is stalled?"  Good question.  So have you defined what you are considering stalled to mean for the purpose of the answer?  Some people will think of stalled to mean something like, "not generating enough lift to oppose the force of weight".  The technical answer is exceeding the critical angle of attack.

@PeterKämpf Why is he wrong the flight test shown in the video clearly shows both wings stalled during the spin.

@DJ319 Don't get confused by the tufts pointing outward on the left wing during a right spin. Centrifugal forces (which become clearly larger with the drooped LE) will make the (still mostly attached!) flow stream towards the wingtip. Nevertheless, especially in the unmodified version the forward 60% of the wing shows attached flow - this is what drives the spin. Flow over the aileron and flaps is secondary and it does not matter much that this region is stalled. The main forces are close to the wing nose.

A spin is an autorotation that requires an asymmetric thrust force to sustain.  This requires the wing span to be anchored at one end by drag, with the other end developing enough thrust to overcome the (rather weak) stabilizing force of the vertical fin and drive that end forward, rotating the plane.  The AOA is highest at the inboard end and decreases as you move outboard due to the higher forward velocity.   At some point along the span, the outer end is unstalled or only semi-stalled and is making at least some amount of lift/thrust.

Yes, both wings are stalled,
but it is mostly about terminology.
Stalls are  NOT immediately all or nothing. i.e. when a wing stalls, it looses a lot, but not all lift. I.e in a stall the wing still reacts somewhat to changes in AOA.
So with both sides of the wing stalled, in a spin, one side of the wing still generates slightly more lift than the other.
Also in most modern aircraft it would be difficult to get into a spin while one wing is stalled and the other one is not.
So yeah, the FAA is right


text:
Which US airport historically has the most days in which VMC prevail?
I live in a part of the country where VMC are starting to become increasingly uncommon, which has led me to wonder - which US airport historically has the most days in which VMC prevail per year?  What sources of information can be used to investigate VMC trends at a given airport?

If those data are available (for the US), then it may be in the Enhanced Traffic Management System (ETMS), but only for major airports. The data are used for instance to predict the airspace capacity (ATC) based on airports conditions and current status. I've seen multiple references from MITRE to ETMS, when talking about VMC/IMC airport performances.

Not enough for a full answer, but https://www.ncdc.noaa.gov/data-access/land-based-station-data/land-based-datasets has archived ASOS and AWOS reports, with which someone would be able to draw some conclusions (though your airport coverage obviously isn't 100%).

https://mesonet.agron.iastate.edu/request/download.phtml has slightly more-direct access to ASOS/AWOS reports, but may be less convenient because of the "network" boundaries.  But might be helpful.

I'd say Mojave is a good candidate. According to this article, they have and average of fewer than 5 non-VMC days per year. It's going to be pretty hard to beat that.

According to NOAA here are the cities with >80% annual sunshine which I've used as a proxy for VMC.  You'll find these cities are all in the desert southwest region of the US where the climate is arid.
SOURCE:  http://www.ncdc.noaa.gov/oa/climate/online/ccd/pctpos.txt
DATA THROUGH 2004

CITY, STATE            ANNUAL % AVG POSSIBLE SUNSHINE        NO. OF YEARS OF DATA
YUMA, AZ               90%                                   42
REDDING, CA            88%                                   10
LAS VEGAS, NV          85%                                   47
PHOENIX, AZ            85%                                   101
TUCSON, AZ             85%                                   53
EL PASO, TX            84%                                   54

To investigate long-term trends you can access historical data at the NCDC website and analyze the data for specific weather reporting stations.

As I'm sure you know, it doesn't have to be sunny to be VMC, so this data is pretty skewed. I can also say that Florida is a very popular training destination too because of the high number of VMC days each year.

@Lnafziger yea, I know. if I find time I'll download the ncdc data and run some statistics with actual vmc conditions.


text:
Is there an instrument departure procedure or minimum performance gradient in the absence of a published standard instrument departure?
Is there an instrument departure procedure or minimum performance gradient in the absence of a published standard instrument departure (SID)?
Departing under instrument flight rules (IFR) in the United States airports sometimes have a SID procedure or alternate take-off minimums which must be observed in order to provide terrain and obstacle clearance.
If there is no SID or alternate take-off minimum published, is there a standard departure procedure to be used in the absence of other procedures? Is there a climb performance gradient which is assumed that provides clearance under such a departure?

Yes, the required obstacle clearance is based on this minimum performance:

Cross the departure end of the runway (DER) at at least 35ft AGL (that's above the DER, not above the airport reference elevation)
Climb to 400ft above the DER before turning
Maintain a climb of at least 200ft per nautical mile (FPNM) until reaching minimum IFR altitude

The minimum IFR altitude is 1000ft above obstacles, or 2000ft in mountainous areas.
This is explained in detail in Chapter 1 of the FAA Instrument Procedures Handbook (starting on page 1-14). The FAA carries out an assessment to see if the airport needs a specific procedure (ODP or SID) and if it doesn't then the standard clearance performance above applies:

If an aircraft may  turn in any direction from a runway within the
  limits of the  assessment area and remain clear of obstacles that
  runway  passes what is called a diverse departure assessment, and  no
  ODP is published.

you can also read the TERPS, beginning page 170 on departures: http://www.faa.gov/documentLibrary/media/Order/8260.3BCHG19.pdf


text:
How should time as safety pilot be logged?
Let's take a classic example. An instrument rated pilot is flying a C-172 and wants to maintain instrument currency. The pilot goes flying and performs some approaches, holding and intercept/tracking to maintain that currency, all while under the hood. In order to maintain a safe condition, a safety pilot is brought along to maintain VFR conditions.
The question is, how should the safety pilot log this time? In this condition the safety pilot is a required crew member, but is this time technically SIC or is it PIC?

The rules for safety pilots are spelled out in 91.109c but don't specifically state how the safety pilot should log the hours.
However, the common consensus I've heard from friends is that you can indeed log safety pilot time as PIC time. The August 2015 Flight Training magazine had an article about this actually. Here is what they said:

What is implied but not specifically stated in the regulation is important. Because safety pilots are responsible for determining the safety of the flight, they can log their right-seat time when the left seat pilot is wearing a view-limiting device as PIC time. This transforms time spent serving as a safety pilot into a truly valuable educational experience that will also expand the flight time totals in your logbook

Edit: I did some more research into this.
PIC time is defined in Part 61.51(e). Safety pilots can qualify as PIC under bullet iii:

(iii) When the pilot, except for a holder of a sport or recreational pilot certificate, acts as pilot in command of an aircraft for which more than one pilot is required under the type certification of the aircraft or the regulations under which the flight is conducted

There are some other things to be aware of when logging PIC time as a safety pilot (in addition to the requirements of 91.109c):

The two pilots must agree that the safety pilot is the acting
PIC.
PIC time may be logged only while the other pilot is
"under-the-hood."
If something goes wrong you are the one that will be held responsible. I wouldnt take the responsibility lightly.
The pilot logging instrument time must record the safety pilots name in their logbook under 61.51(g)(3)

Point 3 is by far the most important.  Imagine logging PIC time for many flights as the safety pilot.  When an accident or incident (airspace infraction) occurs the FAA will ask who the legal PIC was.  No doubt fingers may be pointing to the other pilot.  The FAA would review the logbooks to see if the safety pilot logged PIC time on previous flights to determine who was probably the legal PIC on the flight in question.

In a single-pilot aircraft like the C172 there are basically two scenarios. The key difference between the two is: who is acting as PIC? As background, keep in mind the following:

Acting as PIC and logging PIC are two different things
Per 91.109(c), a safety pilot is required for simulated instrument flight
14 CFR 61.51 has the complete rules on logging PIC and SIC time

Scenario 1: the safety pilot acts as PIC
In this scenario, the safety pilot (SP) acts as PIC while the pilot flying (PF) actually controls the aircraft. That means the SP can log PIC time per 61.51(e)(iii) because they are acting as (emphasis mine):

[...] pilot in command of an aircraft for which more than one pilot is
  required under the type certification of the aircraft or the
  regulations under which the flight is conducted

Meanwhile, the PF can also log PIC time per 61.51(e)(i) as long as they are the "sole manipulator of the controls", i.e. they are actually flying the aircraft.
Scenario 2: the pilot flying acts as PIC
Here, the SP cannot log PIC time. They are not acting as PIC, nor are they the sole manipulator of the controls. However, they are still a required crewmember per 91.109, so they can log SIC time per 61.51(f)(2). The PF can log PIC time as the sole manipulator of the controls.
In practice1, what usually happens is that the pilots agree that the SP will act as PIC. The reason is that it allows both pilots to log PIC time, which is usually desirable for both of them. But that means the SP must be authorized to act as PIC, i.e. valid medical, qualified to act as PIC for the aircraft type, current etc. If the SP is qualified to be a safety pilot but not PIC, then only scenario 2 above is possible.
We have a lot of questions on this site about logging PIC time in various scenarios, so please search and review them carefully before asking a new question that may already have been answered.

1 What really happens most of the time in practice is that the pilots don't discuss any of this and they just both log PIC time for the flight. That's not ideal, but if something goes wrong the FAA will ask "so, who was acting as PIC?". If the pilots can't answer that clearly, things probably won't go so well for either of them. And if the SP logged PIC time without being qualified to do so, it's likely to go even worse.


text:
What are the key differences between controlled and uncontrolled airspace under the FAA jurisdiction?
The FAA utilizes the ICAO definitions for class A-E airspace as controlled and class G as uncontrolled (class F airspace is unused under the FAA). What are the key differences between controlled and uncontrolled airspace in general?

This Document covers all the regs pretty well. But in short the key point on class G (uncontrolled) is

Although ATC has no authority
  or responsibility to control air traffic, pilots should remember
  there are visual flight rules (VFR) minimums which apply
  to Class G airspace.

Class G airspace also holds no specific entry requirements (aside from VFR minimums), no specific aircraft equipment (no radio, no transponder etc) and no minimum pilot certificate requirement (but you still need a license (one of a few varieties) you just don't need anything in addition like the instrument rating requirements for class A). On the contrary glass G airspace (uncontrolled) is not a free for all and you are not free to do as you please in some regards. While ATC has no reach into Class G the FAA does and you are still bound by the regulations the FAA puts forth that apply to class G airspace.
Likewise the definition they provide for controlled is

Controlled airspace is a generic term that covers the
  different classifications of airspace and defined dimensions
  within which air traffic control (ATC) service is provided
  in accordance with the airspace classification.

If you are wondering what the specific purpose of ATC is they define that as well,

The primary purpose of the ATC system is to prevent a
  collision between aircraft operating in the system and to
  organize and expedite the flow of traffic.

You might want to rephrase your comment about "no minimum pilot certificate", it makes it sound like you can go flying in class G with no certificate at all. The document you quoted actually says "no specific requirement", which also applies to class E (and C and D).

The shortest answer is: in uncontrolled airspace, you fly IFR without a clearance (and no clearance is available); in controlled airspace, you cannot fly IFR without a clearance.
That is the only real difference. Everything else (like weather minimums for VFR) follows from that.
BTW Class E is controlled airspace, and has no specific entry requirements for VFR either.


text:
How far ahead will ATC issue runway crossing clearances at controlled FAA airport?
In past years it was common to get a taxi clearance that would include all required runway crossings. This was true even if there were several runway crossings over a long distance.
Now it seems that ATC has limits on how far ahead they can issue runway crossings. What are the ATC limits on taxi clearances that include runway crossings?

I suspect that you're thinking of this procedural change that took effect from June 30th, 2010. Before the change, a taxi clearance could indeed allow you to cross multiple runways, but now you can only be cleared to cross one runway at a time:

This change establishes the requirement that an explicit runway
  crossing clearance be issued for each runway (active/inactive or
  closed) crossing and requires an aircraft/vehicle to have crossed the
  previous runway before another runway crossing clearance may be
  issued. At airports where the taxi route between runway centerlines is
  less than 1,000 feet apart, multiple runway crossings may be issued
  after receiving approval by the Terminal Services Director of
  Operations.

It was a big change at the time (according to what I've read) and it was widely publicized by the FAA and other organizations. The relevant ATC instructions are in the ATC orders 3-7-2:

Aircraft/vehicles must receive a clearance for each runway their route
  crosses. An aircraft/vehicle must have crossed a previous runway
  before another runway crossing clearance may be issued.

The orders have several examples and the corresponding phraseology.

This... It did change, as the OP suspected.

What does the OP acronym mean?

And yes, this is the change I was looking for. Thank you.

@RyanBurnette Original Poster. And you're welcome!

There's no single answer to this: It very much depends on the airport and traffic, time of day, runways in use.
At Gatwick, for example, one runway is basically a permanent taxiway unless the other is closed for any reason - so you'll often be cleared to cross it along with the rest of your taxi instructions. Similarly at San Francisco, only two of the four runways are active at any given moment, so if you have to cross the other two, it wouldn't be unusual to be cleared for them as part of your taxi route.
Then at many smaller airports, they're quiet enough that you can be cleared directly to the gate, including runway crossings.
At the other end of the scale, at JFK at 3pm on a Friday afternoon, you're unlikely to be cleared to cross a runway until you're holding short next to it, and likely only behind an arriving aircraft or 2 8. Or at Schiphol, they seem medically averse to allowing aircraft to cross runways at all, so you'll usually be given a long winded route around a bunch of taxiways rather than crossing the runway.
Most other airports fall somewhere between these, where you'll often be cleared to cross one runway but hold short of another, for example, rather than given a full clearance to the gate. But on another day with an arriving aircraft closer than usual, you'll still be told to hold short.
So unfortunately there's really no better answer than "It depends" - but most ATC's will play it safe (since that's literally their job) and only issue a runway crossing clearance when you're already holding short of the runway. That really doesn't take any more time/effort for them or for you, so it's the safest option and therefore the most common at anything other than quiet airports, or where the operating procedure means you can be cleared to cross a not-in-use runway

SFO typically lands on both 28's while departing both 1's. Otherwise, nice answer.


text:
What does Alternate NA mean on an approach plate?
The ILS Y or LOC Y RWY 3 approach at Raleigh Exec Jetport at Sanford-Lee County Airport (KTTA) in Sanford, North Carolina features the alternate minimums symbol with an NA next to it. Normally this symbol means that the airport has non-standard alternate minimums. What does A NA mean in this case?

Another example is ILS or LOC RWY 33 into KFNL. DPE Andy Munnis uses this approach to illustrate an approach not being authorized for an alternate in his talk about commonly missed items on the instrument oral, beginning around 54:17.

That symbology means this approach cannot be considered when deciding if this airport is a suitable alternate.  This is probably due to the lack of weather reporting or the navaid might not have the required monitoring.  This means the availability of the approach cannot be guaranteed and so you can not rely on it as an alternate.
To file TTA as an alternate you must comply with the the non-standard alternate minimums and you must be legal and equipped to fly either the RNAV (GPS) RWY 03 or RNAV (GPS) RWY 21 procedures.  If the ILS Y RWY 03 is available when you get there, you may fly that procedure instead of the RNAV procedures, you just cannot file it as an alternate on the basis of the ILS.
The wording in the Terminal Procedures legend (PDF) is

Alternate minimums are Not Authorized due to unmonitored facility or absence of weather reporting service

Ye link breaketh.


text:
Will the Boeing 737 be built with a geared turbofan engine?
I've been excited about the potential impact the new Pratt & Whitney PW1000G geared turbofan is going to have on aviation in the form of improved fuel efficiency. There are apparently already plans to retrofit the Airbus A320 with this new geared turbofan engine on the A320neo. The Boeing 737 is a close competitor with the Airbus A320, and could likely also benefit from a retrofit to a geared turbofan. During some hangar chat, I heard it said that there isn't room on the 737 to fit the wider geared engine.
Are there any plans to release a 737 with a geared engine?
If not, could this put the 737 behind its competitors, especially the A320?

The current 737 NG has only one engine choice, the CFM-56. Although this means less choice for the customer, it allows the manufacturer to focus on supporting only one type of engine. The comparable A320 has options for the CFM-56 and the V2500.
The A320neo effectively replaced the CFM-56 with the LEAP-1 and the V2500 with the PW1000G. The 737 MAX will replace the CFM-56 with the LEAP-1, and continue to provide only one engine choice. Note that on the A320neo both engines offer comparable performance overall, and orders are basically split between the two.
Although the 737 MAX is behind the A320neo in sales, it's unlikely that adding another engine choice would make much difference for the considerable expense it would require. The cheapest option is to use the PW1000G, and that still requires a lot of work for the manufacturer to integrate it into the airplane, and then test and certify the 737 with that engine. As with the LEAP-1 and CFM-56, Boeing would prefer to have their own minor version of the engine certified, which adds more expense. There's just no incentive to add another engine option at this point.
A future version of the 737 could possibly use a geared turbofan, but it's too early to tell if such a version will even be built. It certainly remains an option for future designs as engine manufacturers develop the technology further.

No.  All B737MAX are currently planned to use the CFM LEAP-1B engine.
Also, not all A320neo will have the PW geared turbofan engine as it is available with both the PW engine and the CFM LEAP-1A engine.
While the geared turbofan has benefits, it also has its own problems. There were issues that the airlines found significant, one which required 3 minutes warm up before taxi. Here's an article that discusses it.


text:
Why are there no 4-winged airplanes?
With larger wings comes more drag. So why don't large planes have 4 smaller wings instead of 2 very long ones?

Possible duplicate of Why are there no longer any biplanes?

Voting to leave open, the question is not specifically about the classical biplane configuration.

@mins What if each wing has a bigger aspect ratio than the single wing? The bending moments are lower due to shorter span.

@mins That's not how I read it.

Not a large plane, but there have been 4-wing aircraft, such as the SuperMarine Nighthawk

In general, the shorter the wing the higher the drag, the longer the wing the lower the drag. The ideal wing has infinite length - that's what is assumed when you calculate drag of an airfoil instead of a real wing. The reason why drag is reduced when you increase wing length is because induced drag is much, much higher than friction drag.

Why stop at four wings?

With larger wings comes more drag. True, because the drag is a function of the wing area A:
$$ D = C_D \cdot \frac {1}{2} \rho \cdot V^2 \cdot A $$
You need a certain wing area to support the weight of the aircraft. Now the question is: over how many wings will we distribute this wing area? For over 80 years, the answer has been: one wing (the two wing halves make up one wing).
The classical biplane is a thing of the past, however there have been incredibly clever aircraft builders who have looked at the long coupled canard configuration. This one really makes a lot of sense:
 Image source
So to answer your question: there are aircraft with two wings. It's just that people don't buy them, and go for boring old Cessna's instead.

It's not so much that people buy boring 2-winged Cessnas (and Pipers &c), as that they buy old ones.  Mainly because the old ones can be had for 10-20% of the price of new, and not that many people have a spare $250K or so to spend on a new 172.

Is the plane in the picture missing its front gear?

@Burhan: Burt Rutan designed it with a fixed main gear but retractable nose-wheel. However you'd be a brave man (or very very knowledgable) to challenge his design decisions.

Your “drag” expression includes only parasite drag. Adding the term for induced drag sends anything else than single main wing straight into the land of bad ideas. Including most of the Rutan's ones—they were pretty efficient in spite of the unusual layouts, not because of them.

@JanHudec How can a design be efficient in spite of the design?

@Koyovis, not in spite of the design, but in spite of the layout. There are many things that affect the final drag beyond layout and Rutan could do them so well to compensate for the fact that for a stable design, canards are (a bit) less efficient than conventional tail.

@JanHudec I'm still not with you. Induced drag is a function of lift and aspect ratio - per wing.

@Koyovis, actually, induced drag is a function of lift and span. The aspect ratio comes about when you divide by wing area to get the scale-independent lift coefficient. Also, it is not really per-wing: tandem wings with AR 1:20 behave approximately as one wing with AR 1:10, but as two separate wings it will require more structure. And then there is the inherent inefficiency of canards in stable designs—since stall of the canard is limiting factor, the main wing is not used to its full potential, so it needs to be a bit bigger.

@Koyovis and of course you have to take the turbulent air flow coming off the trailing edge of the canard into account when calculating the performance of the wing, with large canards this can be significant.

@jwenting Yes indeed. How turbulent is it though, if the canard has a very high aspect ratio (possible due to reduced structural weight, which is proportional to square of the span) and we're in cruise so induced drag is low?

@Koyovis: Rutan's designs are impressive due to his f**k the FAA" attitude. If you disregard mandated maximum stall speeds, your design will have a more impressive top speed. The list goes on. If compared on an equal footing, his designs are poor.

@PeterKämpf Really? The Voyager was a poor design?

@JanHudec ” Your “drag” expression includes only parasite drag”. No, the expression is used as is for total drag. Induced drag appears in the $C_D$ factor: $C_D = C_{D0} + {C_L}^2/(\pi A e)$

Biplanes are a thing and have been since the earliest days of flight.
However, they fell out of favour because they actually have more drag than a corresponding monoplane.  Not only do you need essentially the same amount of wing but there's also the extra supporting structure.
Alternatively, if you're thinking of two wings one behind the other then the turbulence from the front wing will dramatically decrease the lift of the second.

They may be inefficient, but they sure are good looking planes.

Biplanes and other multi-wing configurations suffer from the airflow coupling of the wings. Specifically, lift is due to deflection of the air downwards by the wing. Another nearby wing now is dealing with deflected air, reducing its possible lift. As mentioned earlier, the real lift function depends on span per unit lift, regardless of how many wing (or horizontal tail) surfaces are involved. A Cessna wouldn't fly with 36 one-foot wings...

Another nearby wing now is dealing with deflected air, reducing its possible lift. The other nearby wing could be mounted at a different angle, to keep Angle of Attack constant.

@Koyovis: Now this other wing will produce much more induced drag. Local flow is pointing downwards, so lift will point backwards. It will be much better to stick that extra pair of wings on the tips of the original wing, even when the resulting wing is heavier than the sum of both.

@PeterKämpf Good point about induced drag.

I think there are could be a couple of reasons why one wing is the norm. Firstly, from an induced drag perspective a longer wing, if structurally possible, will provide less drag than a shorter, fatter one. The longer the aspect ratio (the ratio of wingspan to the cord (width) of the wing) the less effect the wingtip vortices have.
Secondly, as mentioned in another answer, a wing behind another wing sees disrupted flow. How this flow is disrupted could be hard to predict when designing the aircraft

Yes the aerodynamics of a single wing with high aspect ratio are very good. However, the bending moment at the wing root is proportional to the wingspan squared...

But people still overcome that to reap the aerodynamic reward

"With larger wings comes more drag" is not true - the opposite in fact (doubly so since you mean wing length, not area).  Drag is a function based on many things, one being the coefficient of Drag (Cd) which is itself made of up many parts (friction, pressure, and parasitic losses) and which is greatly influenced by the shape of the airfoil sections that make up the wing, along with how the designer works to minimise tip loss vortices.
The more "ends" you have, the worse those are, so aeroplanes only have one wing that lifts (the back does not provide lift, so has negligible tip loss vortices).